Actually the life of a plasma is rated at 60,000 hours, and only THEN does it lose *half* its brightness. So do the calculation as to how many hours a day you'll have the TV on and see what that calculates to in days
Both have their advantages and disadvantages. Some good questions to ask are.... do you have windows behind where you will place the TV? Plasma does tend to have better blacks, but LCD is catching up fairly quickly. Better blacks represent better contrast ratios, better colors, and when you turn out the lights and watch a movie you will actually see black, not dark grey on your TV
The Sony Bravia line is very impressive indeed, but as you noted, very pricey. You might be able to get away with a Enhanced Definition Plasma. At ~42" it's a nice deal. High Definition Plasmas in that size will run around ~$500 more.